I hear good things about Primaloft. Having said that, Primaloft jackets look neither as warm nor as nice as a good down jacket. I've taken my Rab on winter exercises and on the hills all year round and have never suffered from it getting wet.

I have Snugpaks Sasquatch softie jacket, though longer than the Rab expedition it still works well up to -20 which has been really useful in the recent weather. It packs down bigger than a down jacket but not much. I also have a Montane Primaloft jacket and you can't beat it for warmth to weight ratio, it weighs next to nothing. More of a just above freezing jacket to be honest.
Both manage well in the rain, only water resistant but no complaints yet.

Warmth to weight ratio down will always be better than synthetic. However, synthetic is still warm when wet unlike down (not an issue if you're just wearing it in the cold though!). Also if you're going to have it packed up for a long time then synthetic is the way to go as if you keep down stuffed into a compression sack or similar for too long it loses it's loft and therefore it's warmth.
Synthetic jackets have certainly come a long way in recent years though for trying to better down.